Ask Mr. Science
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wobbly telescope

Sunspots, and the rotation of the sun

A while ago, a had taken my cheapy, wobbly telescope, and strapped a stick to it with two hose clamps. This stick extended about a foot beyond the eyepiece, and on the end was a perpendicular piece of wood that could hold a 3x5 index card in place. With this, I could project a 2.5" image of the sun onto the card, and bingo, there was a sunspot. I prepared a stack of index cards with a circle the size of the solar image on it. The trick was then to get the image of the sun exactly in the circle, and then quickly mark the sunspots with a pencil. This required quite a bit of dexterity, what with the wobbly mount, twiddling two knobs on the tripod, and the image of the sun moving so fast. I followed sunspots this way for quite a while, but clearly this was not something that a group of grade schoolers could do accurately without a lot of frustration.

picture of the sunspotter I therefore built an equatorial mount especially for observing the sun. In the picture on the right you can see the black spotting scope, which projects an image onto the little square piece of plywood in the back, which acts as an index card holder. Since the sun dips and rises only 22 degrees above and below the average, such a mount is simpler than a general polar mount for a stellar telescope. If you or your class would like to build one, look here. On it I mounted a spotting scope I long ago bought at a garage sale. The whole thing is leveled - there is a little bubble level glued to the base -, and pointed north using a compass - there is a pencil line on the base pointing to magnetic north (which varies from place to place, and from year to year.) Once that is done, turning the big wheel easily keeps the sun's image aligned.

Once a year at least I get a question about the sun, sunspots, or related things such as the magnetic storms that threaten to cause power grid disturbances right around Y2K, when we can least use them. The sunspotter is sturdy enough for a bunch of 5th-graders to set up and operate.
We talk about sunspots as 'hurricanes on the sun', with their own 11-year season. These storms come and go like earth weather, sometimes lasting days, sometimes weeks. Taking real data and analyzing them is something for higher grades I think. They can find out how fast the sun spins around its axis, and measure the inclination of the orbit of the earth, just with paper, ruler and compass: How to take sunspot data and analyze them


Spring 98, 12 May 99, 17 Nov 99 and every year since...

pop bottle with tire valve  
About the weather - fog in a bottle.

Here in Santa Fe, we live in a desert. I wanted them to understand why this is so, when if you go inland a similar distance from the westcoast of europe, you're not in a desert at all. The story has to do with the fact that along the american westcoast, there is a large mountain range, which forces the air up and causes all its moisture to condense out. Many kids in this 3rd grade class had been to California, and as it turns out, all to coastal cities, and they could all attest to the wet climate there, at least compared to here.

To demonstrate that expanding air cools, and as a result cannot hold on to the moisture it contained, I did this demonstration which sure caught their attention (it was also meant to be an intro to the outdoor session on rockets we will do in a few weeks).
I had a 1-liter clear plastic pop bottle, with a few tablespoons of water in it. I had a rubber stopper with a hole through the middle, a few pieces of plastic tubing, and a valve stem. Look here for more details. All this is put together so that you can pump air into the bottle with your bicycle pump. I had a volunteer hold the bottle. After a few strokes, I shook the bottle so that the water could saturate the air in the bottle at that pressure. Actually, since the air that is pumped into the bottle bubbles up through the water, I don't think the shaking is necessary. Anyway, after a few more strokes of the pump, the stopper let go and the compressed air blew out the bottle with a great bang. The bottle took off, and hit the ceiling, luckily just missing the brand-new light fixtures. It is filled with fog which luckily lasts for quite a while. We did this a few more times, this time making sure that the bottle did not take off. However, every time the cork lets go, there is an impressive and spectacular BANG.

21 April 98


Static Electricity

My friend Charles Brunn had built a Wimshurst machine some years ago, which we had brought in in the past, but it was not always reliable. We decided to build a new one, as well and pretty as we could. The picture shows the current status of the machine, with temporary plywood base and uprights, but finished walnut Leyden jars. Eventually all wooden parts will be walnut and birch.

The machine has a hand crank at the back, and makes sparks between the balls on top. There are all sorts of demos you can do with this.

First, there is the 'like charges repel and opposite charges attract', with little aluminum foil balls on thin wire, which repel each other when charged. Another one demonstrating the same thing is a strip of aluminum foil held between the spark balls will flap back and forth as it touches one ball and picks up its charge, and the goes the other way. Paper held between the terminals does not do the same thing, but the sparks go right through it. Sparks do not go through a credit card (before you pull a random card out of your wallet, check them out - I found that I have several cards that apparently are good enough conductors to happily pass sparks; my guess is that ink, laminating glue and other stuff is to blame.
I found a great electrostatic motor on the web, check out the link below.
The simplest electostatic motor is made from a circle of aluminum foil. If you put a few radial creases in the disk, you can balance it on top of a spike so that it can rotate freely. Make cuts around the perimeter so that points are formed that point in one direction around the perimeter. When charged, charge spays off the points, pushing the disk around in the opposite direction. It really goes, and the kids love it. Also, we did another one I picked up from one of the websites, consisting of 2 pie plates, held apart by a transparent cylinder (a plastic cup with the bottom cut off). Little plastic chips dance up and down in it, but snippets of aluminum foil really hop. We managed to charge up some kids to where their hair stood up a little. All in all, one of the more fun science hours. There are plenty of other things you can do, but I only have an hour.

April 98 ... March 2007


How come ants can carry loads 20 times their body weight?

I like this question because it is the simplest example of dimensional analysis, and relates to things kids have already observed themselves. Here's what I did:

-------------- What you need:
(Of course you can modify all this, but this is what I had in the house, and the numbers worked out very nicely with these items)
  • Clay, about 400 grams of it.
  • A scale, in grams.
  • A small ruler, in cm.
  • A sharp knife, or a wire cutter.

I had prepared 2 lumps of clay of 200g each. As we started talking about how they've all seen ants carrying away big cookie crumbs, and many have seen on TV parasol ants carrying big parts of leaves, I fashioned a mouse-like animal out of one of the lumps. When I was done, I weighed it, and of course it was 200g. Then I measured its length, which was (surprise) 10 cm. Now we have to measure how strong our clay mouse was. Since muscle strength depends on how many muscle fibers you can string between two attachment points, you have to measure the cross sectional area of a muscle. Here the knife comes in. You cut the mouse squarely in two (to the protesting cries of the young onlookers), and squeeze the circular cross section into a square, so that you can easily measure the area. If all goes well, this is 4x4 cm2. We called this first mouse Alice, and wrote her statistics on the board.

three clay mice
Alice, Bob and Cathy. Click here for a bigger picture and a more detailed description.
In the next step, the object is to make a mouse, called Bob, which is half the size of Alice. He is half as long, half as high and half as wide as Alice, so I cut the second cube of clay, which had the same weight as Alice, halfway through each of the faces, thereby subdividing it into 8 smaller cubes. Pick one of the cubes and fashion a mouse out of it as before. When this fellow is put on the scale, he weighs in at 25 grams, and he turns out to be 5 cm long. Again ignoring the children's pleas, I cut him in half, and squeeze the cross section into a square shape to measure his muscle strength, which turns out to be proportional to 2x2 cm2.

I play the game one more time, using another of the cubes we cut in step 2, and go through the process one more time, producing an even smaller and cuter mousie which got called Cathy.

The statistics for this trio are written on the board, and shown here:

  Alice Bob Cathy
weight 200 grams 25 grams 3 grams
strength 16 4 1
length 10 cm 5 cm 2.5 cm

The thing to note is that even though the length, weight and strength all go down from Alice to Bob to Cathy, they don't go down in the same way. Alice is 4 times as long as Cathy, but is 16 times as strong and weighs 64 times as much!

Suppose now that for each unit of strength, Alice can carry a load of 12.5 grams. Since he has a strength of '16', he can carry a load of 16x12.5=200 grams. In other words, he can carry a load equal to his own body weight. Now look at Bob: his muscle strength is rated a '4', so he can carry only 4×12.5 = 50 grams. However, this is twice his own body weight. Finally Alice can carry only 12.5 grams, but this is four times her own body weight!

  Alice Bob Cathy
can carry 200g 50g 12.5g
times own weight 1x 2x 4x

So, smaller animals can lift more stuff, relative to their own weight, even though their muscles are made of the same stuff. The trick is that length is measured in one dimension, whereas strength is proportional to length squared (=surface area), and weight is proportional to length to the third power (=volume).

Going the other way, if an ant were magically enlarged to the size of a sheep, she would not be able to stand up, and a mouse scaled up to the size of an elephant would break all it's bones under it's own weight. The strength of the bones, proportional the the cross section, needs to be larger. Therefore elephant bones are a lot thicker than simply enlarged mouse bones.

May 98, Feb 99, March 2005, October 2007

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Last update 15 May 2005 - HvH
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